1. Field of the Invention
The present invention relates to a method and arrangement of adaptively multiplexing a plurality of video signals after being digitally encoded, and more specifically to such a method and arrangement wherein a transmission bit rate of each video signal is effectively determined using a look-up table.
2. Description of the Prior Art
It is known in the art to adaptively multiplex a plurality of video signals after digital encoding and then transmit the multiplexed signals over a transmission path (channel). One of such techniques has been disclosed in a paper entitled "Statistical Performance Analysis of an Interframe Encoder for Broadcast Television Signals" by Toshio KOGA, et el., IEEE Transactions on Communications, Vol. COM-29, No. 12, December 1981, pages 1868-1876.
Before turning to the present invention it is deemed preferable to briefly discuss the known technique with reference to FIG. 1 which corresponds to FIG. 9 given in the prior art on page 1873.
Three channels of video and audio signals, denoted by V1-A1, V2-A2 and V3-A3, are respectively applied to encoders 10a-10c which in turn apply control signals CONT1-CONT3 to an adaptive bit sharing multiplexer (ABS-MUX) 12. Each of the Control signals CONT1-CONT3 indicates a memory occupancy value of a buffer (not shown) provided in the corresponding encoder (10a, 10b or 10c). The multiplexer 12 determines bit rates to be assigned to the encoders 10a-10c with the total bit rate kept constant at 60 Mbits/s. The average bit rate per channel is 20 Mbit/s in the three-channel arrangement as shown.
The bit rate assignment determination is performed every 153.6 .mu.s (viz., one adaptive bit sharing (ABS) frame time). The ABS frame (hereinlater may be referred to as a frame) is composed of 18 sub-frames each of which includes 512 bits. The bit rate assignment is implemented by changing the number of sub-frames assigned to each channel in one frame. For example, in the case of the three-channel multiplexing, the number of sub-frames assigned to each channel is selected from 5, 6, 7 and 8 with the total number of the sub-frames allocated to the three channels being kept constant at 18. Since one frame has 18 sub-frames in this particular case, the bit rates to be assigned to the three incoming signal channels are combinations selected among 16.7 Mbite/s, 20.0 Mbits/s, 23.3 Mbits/s and 26.7 Mbits/s, which respectively correspond to 5, 6, 7 and 8 sub-frames.
An adaptive bit sharing demultiplexer 14 is arranged to receive a multiplexed data Dm transmitted over a transmission path 16, generates three data D1'-D3' which are replicas of the original data D1-D3, and extract clocks CLKS from the received data. The reproduced data D1'-D3' are respectively applied to decoders 18a-18c together with the clocks. The decoders 18a-18c produce three pairs of video and audio signals V1'-A1', V2'-A2' and V3'-A3' corresponding to the original signals applied to the decoders 10a-10c.
By adaptively selecting three of the four predetermined bit rates and assigning them to the three channels every frame, the prior art is advantageous in simplifying the bit rate assignment operations. Thus, the overall operation time at the transmitter can effectively be reduced.
However, this prior art has encountered the problems set forth below in that the number of bit rates available is restricted to four.
For the sake of description it is assumed that: (a) each of the two video signals V1 and V2 provides picture information including rapid motions of images and/or subject to frequent switching of picture scenes (for example) and thus (b) each of the signals V1, V2 is preferable to be transmitted at the maximum bit rate (viz., 26.7 Mbits/s) for exhibiting reasonable reproduced picture quality. In this instance, the total of the two bit rates assigned to V1 and V2 is 53.4 (=26.7.times.2) Mbits/s. This means that the remaining bit rate is 6.6 Mbits/s. Therefore, even if the third video signal V3 provides still pictures and hence can be transmitted without degrading signal quality at less than 6.6 (=60.0-53.4) Mbits/s, the video signal V3 has to be transmitted at 16.7 Mbits/s (viz., the minimum bit rate). Accordingly, one of the video signals V1, V2 is undesirably transmitted at 23.3 Mbits/s while the other is transmitted at 20 Mbits/s. Thus, each of V1 and V2 is transmitted at the bit rate lower than that preferable. Accordingly, the reproduced signals of V1 and V2 are inevitably degraded in terms of signal quality.